The invention relates to a picture display device having a display tube with a device for generating electron beams at the rear and a phosphor screen at the front, an electromagnetic deflection unit mounted around part of the display tube for deflecting electron beams across the display screen includes a line deflection coil and a field deflection coil. A compensation coil system having at least one compensation coil for generating a magnetic dipole field, is oriented in such manner and in operation in energisable in such manner that, measured at a predetermined distance from the operative display device, the magnetic (interference) field generated by the line deflection coil is at least partly compensated.
A picture display device of this type having means for compensating (line deflection coil) stray fields is known from EP-A No. 220777.
Recently more stringent standards have been introduced for certain types of picture display devices, notably for monitors, with respect to the magnetic interference field which they may produce around them. An important source of magnetic interference fields is the line deflection coil because it is operated at radio frequency currents (frequencies in the range of 10 to 10 kHz) as contrasted to the field deflection coil. It is impossible to design a satisfactory operating deflection coil that produces no stray field. If the stray field were to be eliminated by means of a protective shield, such a shield would only be effective if the combination of display tube and deflection unit were also shielded on the display screen side. The abovecited patent application describes the use of a compensation coil system for eliminating the line deflection stray field at a larger distance, which system, when energized, generates a compensating magnetic dipole field. This dipole field can be obtained by energising one coil whose turns are mainly in one flat plane (a current loop), which coil has the correct number of turns, the correct surface area and the correct orientation. The fact that the spatial position of the compensating dipole moment deviates from that of the deflection unit (which is positioned more to the front) makes little difference at a larger distance (&gt;3 m). Energising can be effected by arranging the compensation coil in series with or parallel to the line deflecting coil. This dipole field can be obtained in an alternative manner by energising two current loops which are positioned on the outside of the line deflection coil with two main portions of their length extending at least approximately parallel to the tube axis on facing sides thereof, which current loops have the correct number of turns, the correct surface area and the correct orientation. Energising may also be effected by arranging the compenstion coils formed by the current loops in series with or parallel to the line deflection coil. For a compensation at smaller distances (for example, 0.5 m from the deflection unit) it is desirable to generate a compensating fourpole field with the compensation coil system. This fourpole field can be generated, for example, by means of two coils or coil portions which are positioned symmetrically relative to the plane of symmetry of the line deflection coils and perpendicularly to the tube axis.
It has been calculated that the radiation field of a line deflection coil at a distance of 0.5 m can be suppressed, for example, by a factor of 20 with a compensation coil system generating a dipole-fourpole field, thus just complying with the current requirements.
It has been found that a compensation coil system dimensioned and energised in order to realise the above-described effect often suppressed the radiation field by not more than a factor of 10, that is to say, only a partial compensation occurred after the system had been placed on a combination of display tube and deflection coil.